This invention relates to high-voltage gas-insulated power transmission systems, and more specifically relates to a novel insulator for supporting the central elongated conducting bus within the interior of a gas-filled grounded conductive cylindrical housing.
A typical gas-insulated system involves a coaxial geometry in which a cylindrical conductor is located coaxially inside a cylindrical enclosure. The conductor is supported by insulators, fabricated from solid dielectric, placed at discrete intervals along the system. The insulator may assume a variety of shapes such as disc, post, cone, or perforated disc and the optimum design depends on the particular application.
Typical support insulators of disc shape and of conical shape are shown in U.S. Pat. Nos. 3,813,475 in the name of Cronin and 3,639,671 in the name of Clark, each of which are assigned to the assignee of the present invention.
Prior art insulators experience a basic problem due to the presence of gas such as sulfur hexafluoride under about three atmospheres of pressure in the dielectric-stress series with a solid dielectric material, which leads to dielectric overstressing of the gas gap, with a consequent potential for flashover. For example, in a disc-type insulator, there will be very short gas gaps between the interior diameter of the insulator and the outer diameter of the conductor supported by the insulator since it is almost impossible to get a perfect fit of the insulator on the conductor. If the disc insulation material has a dielectric constant of 5, the stress in the short gas gap radially beneath the dielectric is five times the stress in the gas at regions removed from the insulator. This can lead to either a flashover along the surface of the disc or a puncture of the solid material.
The above problem is generally overcome by placing a metal insert in the inner diameter of the insulator to short circuit any gas gap to the central conductor, and eliminate any local stress concentration. However, making insulators with conductive inserts is difficult and contributes to the high cost of such insulators. All the problems experienced above for disc insulators were also experienced for conical isulators which, however, offer a longer creepage path than the disc-shaped insulator.